This invention pertains to a process to manufacture a sintered part from powdered material, in particular sinterable metallurgical powder.
The manufacture of sintered parts by pressing a metallurgical powder and then sintering is basic knowledge. When the powder is pressed into a so-called green compact, the quality of the compact depends for one thing on attaining as even a compaction of the powder as possible and on the other hand the geometry of the part must be designed such that the shaping can be carried out with as simple pressing tools as possible. Moreover, the requirement exists in that the pressed green compact can be removed from the press form. Many times, however, the functional requirements of the geometry of the finished part can not be accomplished using a press process if, for example back-tapers, notches running perpendicular to the pressed direction or external contours are present that do not allow an even compaction. To some extent, the problems can be solved by assembling the finished part from two or more sections pressed and sintered individually or by producing a raw part by pressing and sintering. This raw part must then be finished in a machine-shaping process. To construct a part made up of a number of element sections cannot always be accomplished. Machining of a finished sintered part is cost intensive, especially when used in volume production.
A process to manufacture prototypes is known from DE-A-196 36 524 in which a green compact is formed as a basic form of the part in a first single-stage basic forming process from a metal powder that contains binders. Pressure and/or heat are used here. In at least one other material-removal forming process, the green compact is then provided with the desired final form of the part and it is then sintered. The working of a green compact using material-removal shaping processes to produce the final form to be sintered is not applicable for volume production due to the high unit costs.
In order to combine a shaping process with the basic forming process using pressing technology to manufacture a sintered part, a process is described in EP 826 449 in which a green compact is formed in its final form from powdered material using a number of special punches that follow in sequence in a pressing tool. Right at pressing, staged cross sectional contours can be applied with different material thicknesses such as wheel hubs and rims. The prerequisite is that the part""s geometry must have no back-tapers so that it can be removed from the pressing tool again after pressing.
In principle, however, this process can be used for any geometry that has no back-tapers if the pressing tool is adjusted to the contours accordingly. Nonetheless, it has been shown that only for bodies with surfaces that are directed essentially perpendicular to the direction of motion of the pressing tools can an even compaction be attained. As soon as the part to be produced has geometries deviating from this basic condition, the process described runs up against technical limits.
In particular, at the edges and bosses of the part to be produced, areas with less material density can arise due to the low flowability of the powder. This can result in material errors when sintering is subsequently performed such as tears or breaks. In the same manner, overloads and thus breaks can occur at these types of exposed points on the pressing tool.
For parts whose contours or geometries have section that can not be produced using an axially moving pressing tool, either a complicated, a sectional pressing tool is required, for example having lateral slides as well, or it is necessary to do a special process after the basic forming process. In material-removal work, the corresponding geometries or back-tapers on the part are done through machining to attain the final desired form of the part.
The objective of this invention is to create a process that avoids the disadvantages described above.
The objective of met by means of a process to manufacture a sintered part from powdered material, in particular from a sinterable metallurgical powder, in which, first of all, a green compact is pressed, forming a basic form of the part, and in which the desired final form of the part is produced by at least one subsequent non-machined modification of sections on the basic form of the part, which is then finish-sintered. This process offers the advantage for a number of geometries in that the green compact can be made in a relatively simple pressing tool designed for an even compaction. It is useful if the geometry of the basic form of the part approximates the geometry of the final form of the part as much as possible. The specialized final form of the part is then accomplished by means of at least one more special modification of the affected sections of the green compact using another modification tool.
In an embodiment of the process according to the invention, it is provided that the sections to be modified are subjected to pressure in special modification tools. Here, areas that were less compacted in the first pressing step can be compressed again subsequently. Special geometries in the sections of the basic form of the part that are not formed in the first pressing step, or are difficult to form, can be modified. The modification tools are equipped with pressure and counterpressure means. In this method of processing, an amount of isostatic pressure can be transferred to the section to be modified such that even with very brittle material it is still possible to deform it. By modifying the affected sections of the green compact, the final form of the part is produced that can be then sintered.
According to the geometry of the part, it is even possible to even raise the material density in sections by means of the subsequent modification and thus to attain an additional strength in these sections in the finished sintered part.
In an embodiment of the process according to the invention, the modification can be done by means of pressing and/or rolling. The modification can in particular be done in steps, wherein individual contours, such as back-tapers can be produced on the final form of the part through at least one modification stage.
According to the invention, it is also provided that the modified depth increases in steps. In the process, larger modification work can be applied without destroying the material matrix.
In another advantageous embodiment of the process, the green compact is pre-sintered prior to at least one modification to raise the green strength. This joining of the powdered, pressed powder material, called pre-sintering, is preferred to be done at a lower temperature than the high[-temperature] sintering that leads to the final form of the part. The pre-sintering is done in such a manner that it is still possible to do more modification work on the part. By pre-sintering, the inner grain structure of the formed part in the sections that are already in their final form is largely retained when the [other] sections are modified and an increased pressure can be applied to these sections for modification.
According to the invention, the part can be calibrated as a green compact prior to sintering and/or as a solidified part after sintering. It is particularly also provided to apply at least a part of the modification work through calibrating. By this calibration, the surface can be qualitatively improved, as can the grain structure of the part. It is particularly possible to remove ridges and/or peaks or sharp edges.